82 



PRACTICAL BOTANY. 



I o 



QG 



an abundant gelatinous substance. This plant shows finely 

 the typical multiplication by cell division. The hyaline 



envelope holds the cells in posi- 

 tion, and besides single cells, two, 



four, and eight celled colonies 

 are arranged with great regu- 

 larity. Tetraspora may be found 

 on the bottom and on rocks in 

 clear, swift-running streams. 



Pediastrum occurs in colonies 

 made up usually of sixteen cells. 

 These form a flat disk which 



floats on the water. 

 BO 



FIG. 40. Tetraspora lubrica. 

 The transparent gelatinous 

 substance which surrounds 

 the cells and holds them in 

 position is not shown in the 

 figure. (From Gray.) 



Hydrodictyon, or the 

 Water Net, is common 

 in some places. It at- 

 tains a considerable size, 

 sometimes being six or 

 seven inches in diame- 

 ter. It is made up of 

 a green open network 

 of filaments or tubular 

 cells. 



The asexual reproduc- 

 tion is brought about by 

 the formation of a great 

 number of swarm spores 

 in certain of the tubular 

 cells. If the cell is ob- 



FiG.41. Pediastrumgranulattim. (X400.) 

 A, a disk of cells adhering to one an- 

 other ; at f/, the innermost layer of the 

 wall of a cell is just issuing from the cell, 

 and contains the daughter cells formed 

 by division of the green protoplasm ; at 

 t, various states of division of the cells ; 

 sp, slits in the walls of cells which have 

 discharged their contents. B, the inner 

 lamella of the wall of the mother cell 

 disengaged from the cell and much en- 

 larged ; b contains the daughter cells 

 #, which are in lively swarming motion. 

 C, the same family of cells four hours 

 and a half after its birth; four hours 

 after the small cells have come to rest 

 they have arranged themselves into a 

 disk, which is already beginning to de- 

 velop into such a one as A. (From 

 Goebel, after A. Braun.) 



